Ink set, and recording method and recorded matter using the same

ABSTRACT

The present invention provides an ink set comprising at least two or more pigment inks different in color, wherein differences in average particle size between the inks different in color are 30 nm or less, the inks each have a sedimentation rate, as indicated by equation (I), of 15% or less, and differences in the sedimentation rate between the inks different in color are 4% or less: 
       Sedimentation rate(%)=( S   0   −S   1 )/ S   0 ×100  (I) 
     wherein S 0  represents an “initial ink concentration”, which is an absorbance at a wavelength of λmax (or at 500 nm when λmax is not detected) at the time when UV spectral characteristics are measured for a 1000-fold diluted solution of the ink; and S 1  represents an “ink concentration after sedimentation”, which is an absorbance at a wavelength of λmax (or at 500 nm when λmax is not detected) at the time when UV spectral characteristics are measured for a 1000-fold diluted solution of a supernatant obtained by centrifuging the ink at 16500 G at 1.60×10 7  g·sec.

This application is a continuation application of U.S. Ser. No.11/392,985 filed on Mar. 29, 2006, which claims priority to JapanesePatent Application No. 2005-094841 filed on Mar. 29, 2005; the contentsof which are hereby incorporated by reference into the presentapplication.

FIELD OF THE INVENTION

The present invention relates to an ink set, and more particularly to anink set suitable for use in an ink jet recording system.

BACKGROUND OF THE INVENTION

The ink jet recording system is a printing method in which droplets ofan ink are allowed to fly and adhere to a recording medium such as paperto perform printing. With recent innovative improvements in inkjetrecording technology, the ink jet recording method has also come to beused in the field of high-definition image recording (printing) whichhas hitherto been achieved in the fields of photography and offsetprinting. Accordingly, high-quality recording has been required not onlyto plain paper and paper exclusive for ink jet recording (matte seriesand glossy series) which are generally used, but also to a recordingmedium such as paper for printing.

As the ink used in ink jet recording, there has been generally known anink composition containing a coloring material acting as a coloringagent, water, a water-soluble solvent, a surfactant and the like.

As the coloring material, there is used a dye or a pigment, andparticularly, a dye is used in a color ink in many cases, because of itshigh color saturation, transparency and high solubility in water.However, the dye is generally insufficient in terms of light resistanceand gas resistance, and also has a problem in water resistance, becauseit is soluble in water. Accordingly, recorded matter in which recordinghas been made with the ink using the dye has the disadvantage of beingpoor in keeping properties of recorded images. In contrast, the pigmentis a coloring material excellent in such light resistance, gasresistance and water resistance, so that development of pigment inks inwhich this coloring material characteristic is put to good use has beenadvanced. For example, there have been proposed an aqueous pigment inkin which the pigment is dispersed with a surfactant or a polymericdispersing agent, an aqueous pigment ink using a self-dispersing pigmentin which a water-dispersible functional group has been imparted to asurface of the pigment, and an aqueous pigment ink using fine coloredparticles in which the pigment has been coated with a water-dispersibleresin.

In such pigment inks, the pigment particles (including the fine coloredparticles containing the pigment) are stably dispersed in ink solutions.However, when a ink cartridge is allowed to stand for a long period oftime, or when the ink is left as being stored in an ink storage chamberof a printer, the pigment particles are gradually sedimented in theinside of the ink storage unit to generate a gradient in pigmentparticle concentration in the ink solution. For that reason, thedifference in density is generated between ink density printed with theink in an upper portion of the ink storage unit and that printed withthe ink in a lower portion thereof. Accordingly, particularly in arecording method using two or more kinds of different colors, there hasbeen the problem that not only the difference in density in monochromeis generated, but also the hue in color mixing changes to lose the colorbalance of a printed image to be originally obtained, resulting indeteriorated image quality.

Then, in order to prevent such deterioration of color balance, there hasbeen proposed a method of specifying the degrees of sedimentation ofrespective inks of an ink set used for forming a color image, such as ayellow ink, a magenta ink and a cyan ink (see patent document 1).

Further, there has been proposed a method of substantially matching thedegrees of sedimentation of respective inks by adjusting the compressionvoid volume of a porous carrier impregnated with each ink in an inkcartridge to the sedimentation velocity of each ink (see patent document2).

Patent Document 1: JP-A-2000-355667

Patent Document 2: JP-A-2001-328282

However, the degrees of sedimentation shown in patent document 1 arespecified by combining the inks each selected increasing or decreasingthe degree of sedimentation thereof according to a definite rule.According to this method, however, there has been the problem ofinsufficient improvement of deterioration of the gloss balance or colorbalance in image quality (photograph quality) on glossy media.

Further, the method of matching the degrees of sedimentation ofrespective inks by adjusting the compression void volume of a porouscarrier impregnated with each ink in an ink cartridge to thesedimentation velocity of each ink is a means effective only for the inkcartridge equipped with the porous carrier. Accordingly, there has beenthe problem of the lack of flexibility.

SUMMARY OF THE INVENTION

The invention has been made in view of the above-mentioned problems.

Accordingly, and an object thereof is to provide a pigment ink set whichcan form images having excellent glossy texture on various recordingmedia, particularly on a glossy medium, and which does not inducedeterioration of image quality (deterioration of glossiness or colorbalance) caused by sedimentation of pigment particles in a pigment inkand is particularly suitable to use for an ink jet recording system.

Other objects and effects of the invention will become apparent from thefollowing description.

As a result of extensive studies, the present inventors have obtained afinding that a pigment ink set comprising at least two kinds of pigmentinks different in color, which is characterized in that the differencein average particle size between the inks different in color is 30 nm orless, that the sedimentation rate for each of the inks is 15% or less,and that the difference in sedimentation rate between the inks differentin color is 4% or less, can solve the above-mentioned problems. Theinvention is based on such a finding, and the constitution of theinvention is as follows:

(1) An ink set comprising at least two or more pigment inks different incolor, wherein differences in average particle size between the inksdifferent in color are 30 nm or less, the inks each have a sedimentationrate, as indicated by equation (I), of 15% or less, and differences inthe sedimentation rate between the inks different in color are 4% orless:

Sedimentation rate(%)=(S ₀ −S ₁)/S ₀×100  (I)

wherein

S₀ represents an “initial ink concentration”, which is an absorbance ata wavelength of λmax (or at 500 nm when λmax is not detected) at thetime when UV spectral characteristics are measured for a 1000-folddiluted solution of the ink; and

S₁ represents an “ink concentration after sedimentation”, which is anabsorbance at a wavelength of λmax (or at 500 nm when λmax is notdetected) at the time when UV spectral characteristics are measured fora 1000-fold diluted solution of a supernatant obtained by centrifugingthe ink at 16500 G at 1.60×10⁷ g·sec;

(2) The ink set described in (1), wherein pigment particles in theabove-mentioned pigment ink have an average particle size ranging from50 to 150 nm;

(3) The ink set described in (1) or (2), wherein the above-mentionedpigment ink contains a pigment in an amount of 0.5 to 8% by weight;

(4) The ink set of any one of (1) to (3), wherein the above-mentionedpigment set contains at least a yellow ink, a magenta ink and a cyanink, the yellow ink contains at least one pigment selected from thegroup consisting of C.I. Pigment Yellow 74, 109, 110, 128, 138, 147,150, 155, 180 and 181, the magenta ink contains at least one pigmentselected from the group consisting of C.I. Pigment Magenta 122, 202,207, 209 and C.I. Pigment Violet 19, and the cyan ink contains at leastone pigment selected from the group consisting of C.I. Pigment Blue 15,15:1, 15:2, 15:3. 15:4 and 16;

(5) The ink set described in any one of (1) to (4), wherein theabove-mentioned ink set contains a dispersion in which the pigment isdispersed in water with a water-insoluble resin;

(6) A recording method comprising allowing the ink set described in anyone of (1) to (6) to adhere to a recording medium to perform printing;

(7) An ink jet recording method comprising ejecting droplets of the inkdescribed in any one of (1) to (6) and allowing the droplets to adhereto a recording medium to perform printing; and

(8) Recorded matter recorded by the recording method described in (6) or(7).

DETAILED DESCRIPTION OF THE INVENTION

The invention will be illustrated in detail below based on preferredembodiments thereof. The ink set of the invention is a pigment ink setcomprising at least two kinds of pigment inks different in color, whichis characterized in that the difference in average particle size betweenthe inks different in color is 30 nm or less, that the sedimentationrate for each of the inks is 15% or less, and that the difference insedimentation rate between the inks different in color is 4% or less.

It is clear to those skilled in the art that the sedimentation rate ofthe ink is adjustable by controlling the kinds and amounts of pigmentand dispersing agent for dispersing the pigment, the particle size ofpigment particles, the kinds and amounts of other respective componentsconstituting the ink, and the like.

The average particle size in the invention is the volume averageparticle size measured by the use of a particle size distributionanalyzer of a dynamic light scattering system, a laser diffractionscattering system or the like for an original ink solution or a sampleobtained by diluting the ink with pure water to a concentration mostsuitable for the measurement. Specific examples of means for measuringthe particle size include UPA-150, UPA-EX150 and MT3300EX manufacturedby Microtrac, Inc., and LA-300 and LA-950 manufactured by Horiba, Ltd.

Further, the sedimentation rate in the invention is determined accordingto the following equation (I):

Sedimentation rate(%)=(S ₀ −S ₁)/S ₀×100  (I)

S₀: Initial ink concentration=Absorbance at a wavelength of λmax (500 nmwhen λmax is not detected) at the time when UV spectral characteristicsare measured for a 1000-fold diluted solution of the ink

S₁: Ink concentration after sedimentation=Absorbance at a wavelength ofλmax (500 nm when λmax is not detected) at the time when UV spectralcharacteristics are measured for a 1000-fold diluted solution of asupernatant obtained by centrifuging the ink at 16500 G at 1.60×10⁷g·sec

Here, for the measurement of UV spectral characteristics, aspectrophotometer is used, and specific examples thereof include U-3000and U-3300 manufactured by Hitachi, Ltd.

A centrifugal separator is used for the sedimentation, and any apparatusmay be used as long as a revolution of about 10000 rpm can be obtained.A rotor is appropriately selected to perform adjustment so that thedesired centrifugal force is applied to the ink. Specific examples ofsedimentation tests include, for example, a method of performingcentrifugation using Hitachi himac CR20B2 manufactured by Hitachi, Ltd.,equipped with RPRS10 as the rotor, at 1.60×10⁷ g·sec. The sedimentationrate of the ink is determined by an accelerated test of sedimentationobtained by such a method.

Respective constituents of the ink set will be illustrated in detailbelow.

Pigment

The pigment ink constituting the ink set of the invention contains apigment as a coloring agent, and either a known inorganic pigment ororganic pigment can be used as the pigment. Specific examples thereofinclude C.I. Pigment Yellow 1, 3, 12, 13, 14, 17, 24, 34, 35, 37, 42,53, 55, 74, 81, 83, 95, 97, 98, 100, 101, 104, 108, 109, 110, 117, 120,128, 138, 150, 153, 155, 174, 180 and 198, C.I. Pigment Red 1, 3, 5, 8,9, 16, 17, 19, 22, 38, 57:1, 90, 112, 122, 123, 127, 146, 184, 202 and209, C.I. Pigment Violet 1, 3, 5:1, 16, 19, 23 and 38, C.I. Pigment Blue1, 2, 15, 15:1, 15:2, 15:3, 15:4 and 16, and C.I. Pigment Black 1 and 7.A plurality of pigments can also be used in an ink having one color toform the color.

The particularly preferred ink set is an ink set comprising at least ayellow ink, a magenta ink and a cyan ink, wherein the yellow inkcontains as a pigment at least one selected from the group consisting ofC.I. Pigment Yellow 74, 109, 110, 128, 138, 147, 150, 155, 180 and 181,the magenta ink contains as a pigment at least one selected from thegroup consisting of C.I. Pigment Magenta 122, 202, 207, 209 and C.I.Pigment Violet 19, and the cyan ink contains as a pigment at least oneselected from the group consisting of C.I. Pigment Blue 15, 15:1, 15:2,15:3. 15:4 and 16. This makes it possible to enlarge the range of colorreproduction when color images are formed. As a result, printed imagesof high quality can be obtained.

These pigments are preferably incorporated into the ink as a pigmentdispersion in which they have been dispersed in an aqueous mediumtogether with a dispersing agent such as a polymeric dispersing agent ora surfactant by the use of a ball mill, a roll mill, a bead mill, ahigh-pressure homogenizer, a high-speed stirring type disperser or thelike, or as a pigment dispersion in which the pigments processed toself-dispersion type pigments which are dispersible and/or dissolvablein an aqueous medium with no dispersing agent by bondingdispersibility-imparting groups (hydrophilic functional groups and/orsalts thereof) to surfaces of the pigments directly or indirectlythrough alkyl groups, alkyl ether groups, aryl groups or the like hasbeen dispersed in an aqueous medium.

Examples of the dispersing agents include polymeric dispersing agents,and examples thereof include natural polymer compounds such as glue,gelatin and saponin, and synthetic polymer compounds such as polyvinylalcohols, polypyrrolidones, acrylic resins (such as polyacrylic acid, anacrylic acid-acrylonitrile copolymer, a vinyl acetate-acrylic acidcopolymer and a vinyl acetate-acrylic ester copolymer), styrene-acrylicacid resins (such as a styrene-acrylic acid copolymer, astyrene-methacrylic acid copolymer, a styrene-methacrylic acid-alkylacrylate copolymer, a styrene-α-methylstyrene-acrylic acid copolymer, astyrene-α-methylstyrene-acrylic acid-alkyl acrylate copolymer and astyrene-vinyl acetate-acrylic acid copolymer), styrene-maleic acid-basedresins, vinyl acetate-based copolymers such as a vinyl acetate-ethylenecopolymer, vinyl acetate-fatty acid vinyl ester-ethylene copolymerresins, and salts thereof. The constitution of the copolymers may be anyone of a random type, a block type and a graft type.

Further, examples of the surfactants which are used as the dispersingagents include anionic surfactants such as a fatty acid salt, a higheralkyldicarboxylic acid salt, a higher alcohol sulfuric ester salt and ahigher alkylsulfonic acid salt, cationic surfactants such as a fattyacid amine salt and a fatty acid ammonium salt, nonionic surfactantssuch as a polyoxyalkyl ether, a polyoxyalkyl ester and a sorbitan alkylester.

Of these dispersing agents, particularly preferred is a water-insolubleresin. Specifically, this resin means a resin which comprises a blockcopolymer resin of a hydrophobic group-containing monomer and ahydrophilic group-containing monomer, contains at least a salt forminggroup-containing monomer, and has a solubility of less than 1 g based on100 g of water at 25° C. after neutralization. The hydrophobicgroup-containing monomers include methacrylic esters such as methylmethacrylate, ethyl methacrylate, isopropyl methacrylate, n-butylmethacrylate, isobutyl methacrylate, n-amyl methacrylate, isoamylmethacrylate, n-hexyl methacrylate, 2-ethylhexyl methacrylate, octylmethacrylate, decyl methacrylate, dodecyl methacrylate, octadecylmethacrylate, cyclohexyl methacrylate, phenyl methacrylate, benzylmethacrylate and glycidyl methacrylate, vinyl esters such as vinylacetate, vinylcyan compounds such as acrylonitrile andmethacrylonitrile, and aromatic vinyl monomers such as styrene,α-methylstyrene, vinyltoluene, 4-t-butylstyrene, chlorostyrene,vinylanisole and vinylnaphthalene. They can be used either alone or as amixture of two or more thereof. The hydrophilic group-containingmonomers include polyethylene glycol monomethacrylate, polypropyleneglycol monomethacrylate and ethylene glycol-polypropylene glycolmonomethacrylate, and they can be used either alone or as a mixture oftwo or more thereof. The salt forming group-containing groups includeacrylic acid, methacrylic acid, styrenecarboxylic acid and maleic acid,and they can be used either alone or as a mixture of two or morethereof. Further, a macromonomer such as a styrenic macromonomer havinga polymerizable functional group on one end thereof or a silicone-basedmacromonomer, or another monomer can also be used together.

The water-insoluble resin is preferably used as a salt neutralized withan alkali neutralizing agent such as a tertiary amine such astrimethylamine or triethylamine, lithium hydroxide, sodium hydroxide,potassium hydroxide or ammonia, and one having a weight averagemolecular weight of about 10,000 to 150,000 is preferred in that thepigment is stably dispersed.

Further, for these pigments, the average particle size of pigmentparticles in the ink preferably ranges from 50 to 150 nm, in theviewpoints of storage stability of the ink, prevention of nozzleclogging, color development and glossiness on glossy media.

Furthermore, these pigments are preferably contained in the ink of theinvention in an amount ranging from 0.5 to 8% by weight. When thecontent thereof is less than 0.5% by weight, print density (colordevelopment) is insufficient in some cases. On the other hand, exceeding8% by weight results in the occurrence of defects in reliability such asdeterioration of glossiness on glossy media, nozzle clogging andunstable ejection in some cases.

Ink Composition

In the ink set of the invention, an aqueous pigment ink is preferablyused in which a main solvent of the ink is water, from the viewpoints ofsafety and handling properties. As the water, there is preferably usedpure water such as ion exchanged water, ultrafiltrated water, waterobtained by reverse osmosis or distilled water, or ultrapure water. Inparticular, the use of water which has been sterilized by ultravioletirradiation, the addition of hydrogen peroxide, or the like is preferredin that mold or bacteria can be prevented from being developed to makeit possible to store the ink for a long period of time.

Further, the ink set of the invention is an ink set suitable for an inkjet recording method, and preferably contains a water-soluble organicsolvent having a wetting effect, in order to prevent clogging in thevicinity of a nozzle of an ink jet head. Specific examples thereofinclude polyhydric alcohols such as glycerol, 1,2,6-hexanetriol,trimethylolpropane, ethylene glycol, polypropylene glycol, diethyleneglycol, triethylene glycol, tetraethylene glycol, pentaethylene glycol,dipropylene glycol, 2-butene-1,4-diol, 2-ethyl-1,3-hexane-diol and2-methyl-2,4-pentanediol, saccharides such as glucose, mannose,fructose, ribose, xylose, arabinose, galactose, aldonic acid, glucitol(sorbitol), maltose, cellobiose, lactose, sucrose, trehalose andmaltotriose, so-called solid wetting agents such as sugar alcohols,hyaluronic acids and ureas, alkyl alcohols having 1 to 4 carbon atomssuch as ethanol, methanol, butanol, propanol, and isopropanol,2-pyrrolidone, N-methyl-2-pyrrolidone, 1,3-dimethyl-2-imidazolidinone,formamide, acetamide, dimethyl sulfoxide, sorbit, sorbitan, acetin,diacetin, triacetin and sulfolane. One or two or more kinds of thesewater-soluble organic solvents having a wetting effect can be used, andare preferably contained in the ink in an amount of 10 to 50% by weight,from the viewpoints of securing proper physical properties (viscosityand the like) of the ink, print quality and securing reliability.

Further, in order to properly control penetrating properties of the inkinto recording media and blurring, and to impart drying properties tothe ink, it is preferred that a 1,2-alkanediol and/or a glycol ether iscontained. Specific examples of the 1,2-alkanediols include1,2-octanediol, 1,2-hexanediol, 1,2-pentanediol and4-methyl-1,2-pentanediol. Specific examples of the glycol ethers includeethylene glycol monomethyl ether, ethylene glycol monoethyl ether,ethylene glycol monobutyl ether, ethylene glycol monomethyl etheracetate, diethylene glycol monomethyl ether, diethylene glycol monoethylether, diethylene glycol mono-n-propyl ether, ethylene glycolmono-iso-propyl ether, diethylene glycol mono-iso-propyl ether, ethyleneglycol mono-n-butyl ether, ethylene glycol mono-t-butyl ether,diethylene glycol mono-t-butyl ether, 1-methyl-1-methoxybutanol,propylene glycol monomethyl ether, propylene glycol monoethyl ether,propylene glycol mono-t-butyl ether, propylene glycol mono-n-propylether, propylene glycol mono-iso-propyl ether, dipropylene glycolmonomethyl ether, dipropylene glycol monoethyl ether, dipropylene glycolmono-n-propyl ether and dipropylene glycol mono-iso-propyl ether. On ortwo or more of these solvents can be used, and are preferably containedin the ink in an amount of 2 to 15% by weight, from the viewpoints ofsecuring proper penetrating properties and drying properties.

Furthermore, in order to control wetting properties of the ink torecording media to impart uniform glossiness, and to obtain penetratingproperties into recording media and print stability in the ink jetrecording method, it is preferred that a surface tension adjuster iscontained. As the surface tension adjuster, preferred is an acetyleneglycol-based surfactant or a polyether-modified siloxane.

Example of the acetylene glycol-based surfactants include Surfynol 420,440, 465, 485, 104 and STG (manufactured by Air Products and ChemicalsInc.), Olfine PD-001, SPC, E1004 and E1010 (manufactured by NissinChemical Industry Co., Ltd.), and Acetylenol E00, E40, E100 and LH(manufactured by Kawaken Fine Chemicals Co., Ltd.). Further, thepolyether-modified siloxanes include BYK-346, 347, 348 and UV3530(manufactured by Bic Chemie Corp.). Plural kinds of these may be used inthe ink composition. The surface tension is preferably adjusted to 20 to40 mN/m, and these are contained in the ink in an amount of 0.1 to 3.0%by weight.

Further, the ink of the invention preferably contains a pH adjustor.

As the pH adjustor, there can be used an alkali metal hydroxide such aslithium hydroxide, potassium hydroxide or sodium hydroxide, or an aminesuch as ammonia, triethanolamine, tripropanolamine, diethanolamine ormonoethanolamine. Preferably, the ink contains at least one pH adjustorselected from the group consisting of an alkali metal hydroxide,ammonia, triethanolamine and tripropanolamine, and adjusted to pH 6 to10. When the pH is outside this range, materials constituting an ink jetprinter are adversely affected, or clogging recovery is deteriorated.

Further, there may be added an antifoaming agent, an antioxidant, anultraviolet absorber, a preservative or fungicide and the like, asneeded.

Although can be suitably used for writing things such as pens, stampsand the like, the ink composition of the invention can be more suitablyused as an ink composition for ink jet recording. The term “ink jetrecording system” as used in the invention means a system of ejecting anink composition as droplets from a fine nozzle and adhering the dropletsto a recording medium. The ink jet recording systems will be concretelydescribed below.

As the first method, there is an electrostatic attraction system. Thissystem is a system of applying a strong electric field between a nozzleand an accelerating electrode positioned in front of the nozzle tocontinuously eject the ink in a droplet form from the nozzle and givinga print information signal to deflecting electrodes while the inkdroplets are flying between the deflecting electrodes, therebyperforming printing, or a system of ejecting ink droplets in response toa print information signal without deflecting the course of the inkdroplets.

The second method is a system of applying pressure to an ink liquid bymeans of a small-sized pump and vibrating mechanically a nozzle by usinga quartz oscillator, thereby forcibly ejecting ink droplets. The inkdroplets ejected are electrostatically charged, simultaneously with theejection, and an information signal is given to deflecting electrodeswhile the ink droplets are flying between the deflecting electrodes,thereby performing printing.

The third method is a system using a piezoelectric element, and a systemof simultaneously applying pressure and a print information signal to anink liquid with the piezoelectric element, thereby ejecting ink dropletsto perform recording.

The fourth method is a system of rapidly expanding the volume of an inkliquid by the action of thermal energy, and a system of heating the inkliquid at a microelectrode according to a print information signal toproduce bubbles, thereby ejecting ink droplets to perform recording.

All of the above systems can be used in the ink jet recording methodusing the ink set of the invention.

The recorded matter of the invention is one in which recording has beenperformed using at least the above-mentioned ink set. The use of the inkcomposition of the invention provides this recorded matter as recordedmatter which is excellent in color development and has glossy texture.

Examples

The present invention will be illustrated in greater detail withreference to the following examples, but the invention should not beconstrued as being limited thereto.

Preparation of Pigment Dispersions

Coloring agent dispersions in each of which a coloring agent coated witha water-insoluble polymer was contained as dispersed particles wereprepared by the following methods.

(Synthesis of Water-Insoluble Polymers 1 to 3)

Twenty parts by weight of an organic solvent (methyl ethyl ketone), 0.03part by weight of a polymerization chain transfer agent(2-mercaptoethanol), a polymerization initiator and each monomer shownin Table 1 were put in a reaction vessel in which nitrogen gasreplacement was sufficiently performed, and polymerization was conductedat 75° C. under stirring. Then, 0.9 part by weight of2,2′-azobis(2,4-dimethylvarelonitrile) dissolved in 40 parts by weightof methyl ethyl ketone, based on 100 parts by weight of monomercomponents, was added thereto, followed by aging at 80° C. for 1 hour.Thus, polymer solutions were obtained.

TABLE 1 Water- Water- Water- Composition of Monomer Mixture InsolubleInsoluble Insoluble (% by weight) Polymer 1 Polymer 2 Polymer 3Methacrylic Acid 20 15 20 Styrene Monomer 45 30 40 Benzyl Methacrylate20 Polyethylene Glycol 5 10 Monomethacrylate (EO = 15) PolypropyleneGlycol 10 25 Monomethacrylate (PO = 9) Polyethylene Glycol-Propylene 105 Glycol Monomethacrylate (EO = 5, PO = 7) Styrene Macromonomer 20 15 10

(Pigment Dispersion Y1)

The polymer solution obtained as water-insoluble polymer 1 was driedunder reduced pressure, and 5 parts of the resulting product wasdissolved in 15 parts of methyl ethyl ketone. Using an aqueous solutionof sodium hydroxide, the polymer solution was neutralized. Further, 15parts of C.I. Pigment Yellow 74 was added, followed by kneading with adisperser while adding water.

After 100 parts of ion exchanged water was added to the resultingkneaded product, followed by stirring, methyl ethyl ketone was removedat 60° C. under reduced pressure, and water was partially removed,thereby obtaining an aqueous dispersion of the yellow pigment having asolid concentration of 20% by weight.

(Pigment Dispersion Y2)

The polymer solution obtained as water-insoluble polymer 2 was driedunder reduced pressure, and 6 parts of the resulting product wasdissolved in 45 parts of methyl ethyl ketone. Using an aqueous solutionof sodium hydroxide, the polymer solution was neutralized. Further, 18parts of C.I. Pigment Yellow 128 was added, followed by kneading with adisperser while adding water.

After 120 parts of ion exchanged water was added to the resultingkneaded product, followed by stirring, methyl ethyl ketone was removedat 60° C. under reduced pressure, and water was partially removed,thereby obtaining an aqueous dispersion of the yellow pigment having asolid concentration of 20% by weight.

(Pigment Dispersion Y3)

The polymer solution obtained as water-insoluble polymer 3 was driedunder reduced pressure, and 5 parts of the resulting product wasdissolved in 15 parts of methyl ethyl ketone. Using an aqueous solutionof sodium hydroxide, the polymer solution was neutralized. Further, 15parts of C.I. Pigment Yellow 180 was added, followed by kneading with adisperser while adding water.

After 100 parts of ion exchanged water was added to the resultingkneaded product, followed by stirring, methyl ethyl ketone was removedat 60° C. under reduced pressure, and water was partially removed,thereby obtaining an aqueous dispersion of the yellow pigment having asolid concentration of 20% by weight.

(Pigment Dispersion M1)

Pigment dispersion M1 was obtained in the same manner as in pigmentdispersion Y1 with the exception that C.I. Pigment Red 122 was used inplace of C.I. Pigment Yellow 74.

(Pigment Dispersion M2)

Pigment dispersion M2 was obtained in the same manner as in pigmentdispersion Y2 with the exception that C.I. Pigment Violet 19 was used inplace of C.I. Pigment Yellow 128.

(Pigment Dispersion M3)

Pigment dispersion M3 was obtained in the same manner as in pigmentdispersion Y3 with the exception that C.I. Pigment Red 209 was used inplace of C.I. Pigment Yellow 180.

(Pigment Dispersion C1)

Pigment dispersion C1 was obtained in the same manner as in pigmentdispersion Y1 with the exception that C.I. Pigment Blue 15:3 was used inplace of C.I. Pigment Yellow 74.

(Pigment Dispersion C2)

Pigment dispersion C2 was obtained in the same manner as in pigmentdispersion Y2 with the exception that C.I. Pigment Blue 15:1 was used inplace of C.I. Pigment Yellow 128.

(Pigment Dispersion C3)

Pigment dispersion C3 was obtained in the same manner as in pigmentdispersion Y3 with the exception that C.I. Pigment Blue 15:4 was used inplace of C.I. Pigment Yellow 180.

Preparation of Inks

Respective components were mixed in ratios shown in Table 2, and stirredat room temperature for 2 hours. Then, filtration was performed using amembrane filter having a pore size of 5 μm to prepare respective inks.The amounts added shown in Table 2 are all indicated in percentages byweight, and indicated in solid concentration for the pigmentdispersions. Further, the term “balance” of ion exchanged water meansthat ion exchanged water is added to bring the total amount of ink to100 parts.

TABLE 2 Ink Composition Ink Y1 Ink Y2 Ink Y3 Ink Y4 Dispersion Y130(4.6) Dispersion Y2 40(5.3) Dispersion Y3 30(3.8) Dispersion M1Dispersion M2 Dispersion M3 Dispersion C1 Dispersion C2 Dispersion C3C.I. Pigment Yellow 74 (4.5) (Styrene-Acrylic Resin Dispersion) C.I.Pigment Red 122 (Styrene-Acrylic Resin Dispersion) C.I. Pigment Blue15:3 (Styrene-Acrylic Resin Dispersion) Polymer 1 5 5 Polymer 2 Polymer3 5 Polymer 4 Polymer 5 Glycerol 10 10 15 10 Triethylene Glycol 5 3 51,2-Hexanediol 1 1 2 1 Trimethylolpropane 2 4 2 Urea 3 TEGmBE 2 2 2 22-Pyrrolidone 1 2 1 Olfine E1010 1 0.5 0.7 1 Surfynol 104 0.5 0.3 0.70.5 Triethanolamine 1 1 1 1 EDTA 0.02 0.02 0.02 0.02 Proxel XL-2 0.3 0.30.3 0.3 Ion Exchanged Water Balance Balance Balance Balance AverageParticle Size 110 80 100 130 (nm) Sedimentation Rate (%) 12 10 10 18 InkComposition Ink M1 Ink M2 Ink M3 Ink M4 Dispersion Y1 Dispersion Y2Dispersion Y3 Dispersion M1 30(4.6) Dispersion M2 40(5.3) Dispersion M325(4.3) Dispersion C1 Dispersion C2 Dispersion C3 C.I. Pigment Yellow 74(Styrene-Acrylic Resin Dispersion) C.I. Pigment Red 122 (4.5)(Styrene-Acrylic Resin Dispersion) C.I. Pigment Blue 15:3(Styrene-Acrylic Resin Dispersion) Polymer 1 3 Polymer 2 3 Polymer 3Polymer 4 3 Polymer 5 Glycerol 10 10 15 10 Triethylene Glycol 4 2 51,2-Hexanediol 1 1 2 1 Trimethylolpropane 2 4 2 Urea 3 TEGmBE 2 2 2 22-Pyrrolidone 1 2 1 Olfine E1010 1 0.5 0.7 1 Surfynol 104 0.5 0.3 0.70.5 Triethanolamine 1 1 1 1 EDTA 0.02 0.02 0.02 0.02 Proxel XL-2 0.3 0.30.3 0.3 Ion Exchanged Water Balance Balance Balance Balance AverageParticle Size 100 90 120 155 (nm) Sedimentation Rate (%) 12 10 15 15 InkComposition Ink C1 Ink C2 Ink C3 Ink C4 Ink C5 Dispersion Y1 DispersionY2 Dispersion Y3 Dispersion M1 Dispersion M2 Dispersion M3 Dispersion C125(3.8) Dispersion C2 36(4) 36(4) Dispersion C3 30(3) C.I. PigmentYellow 74 (Styrene-Acrylic Resin Dispersion) C.I. Pigment Red 122(Styrene-Acrylic Resin Dispersion) C.I. Pigment Blue 15:3 (3.5)(Styrene-Acrylic Resin Dispersion) Polymer 1 5 5 Polymer 2 Polymer 3 3Polymer 4 3 Polymer 5 5 Glycerol 10 10 10 15 10 Triethylene Glycol 7 5 55 1,2-Hexanediol 1 1 1 2 1 Trimethylolpropane 2 4 4 2 Urea 3 TEGmBE 2 22 2 2 2-Pyrrolidone 1 2 1 Olfine E1010 1 0.5 0.5 0.7 1 Surfynol 104 0.50.3 0.3 0.7 0.5 Triethanolamine 1 1 1 1 1 EDTA 0.02 0.02 0.02 0.02 0.02Proxel XL-2 0.3 0.3 0.3 0.3 0.3 Ion Exchanged Water Balance BalanceBalance Balance Balance Average Particle Size 80 85 85 100 100 (nm)Sedimentation Rate (%) 8 10 7 11 13 TEGmBE: Triethylene glycol monobutylether EDTA: Disodium ethylenediaminetetraacetate

Using the respective inks prepared in Table 2, the following evaluationswere conducted for ink sets shown in Table 3.

TABLE 3 Ink Set Yellow Ink Magenta Ink Cyan Ink Example Ink Set 1 Ink Y1Ink M1 Ink C1 Ink Set 2 Ink Y2 Ink M2 Ink C2 Ink Set 3 Ink Y2 Ink M2 InkC3 Ink Set 4 Ink Y3 Ink M3 Ink C4 Ink Set 5 Ink Y1 Ink M2 Ink C3Comparative Ink Set 6 Ink Y2 Ink M3 Ink C2 Example Ink Set 7 Ink Y2 InkM3 Ink C3 Ink Set 8 Ink Y4 Ink M4 Ink C5

(Test 1) Average Particle Size:

Using a particle size distribution analyzer (UPA-150 manufactured byMicrotrac, Inc.), the particle size distribution was measured for adiluted solution obtained by diluting 1,000 times each ink with purewater to determine the volume average particle size. The results thereofare shown in Table 2.

(Test 2) Sedimentation Rate:

Using a centrifuge (Hitachi himac CR20B2 manufactured by Hitachi, Ltd.was equipped with an RPRS10 rotor), an ink was injected into asedimentation pipe to 55 g including the sedimentation pipe, andcentrifugation was performed at 16500 G at 1.60×10⁷ g·sec. UV spectralcharacteristics of a 1000-fold diluted solution of the resultingsupernatant and a 1000-fold diluted solution of the ink beforecentrifugation were measured by the use of U-300 manufactured byHitachi, Ltd. to determine the sedimentation rate indicated by equation(I). The results thereof are shown in Table 2.

Sedimentation rate(%)=(S ₀ −S ₁)/S ₀×100  (I)

S₀: Initial ink concentration=Absorbance at a wavelength of λmax (500 nmwhen λmax is not detected)

S₁: Ink concentration after sedimentation=Absorbance of a supernatantafter centrifugation at a wavelength of λmax (500 nm when λmax is notdetected)

(Test 3) Glossiness:

An ink cartridge for an ink jet printer, PX-V600 (manufactured by SeikoEpson Corporation) was filled with each ink, and mounted on the ink jetprinter, PX-V600. Then, printing was carried out on EPSON Photo Paper<Gloss> (trade name: manufactured by Seiko Epson Corporation) at aresolution of 1440 dpi to form print images of 5 cm×5 cm patch patternsat the ink ratios (a) to (o) shown below, respectively, in one sheet ofA4 paper at 70% duty for (a) to (g) and at an intermediate density of50% duty for (h) to (o). For each patch, the 20° glossiness (G₂₀) wasmeasured by the use of a glossmeter, GM-268 (manufactured by KonicaMinolta Sensing, Inc.), and judgment was made from the results of theaverage value of the glossinesses of (a) to (j) on the basis of thecriteria shown below:

(a) Yellow 100%

(b) Magenta 100%

(c) Cyan 100%

(d) Black 100%

(e) Yellow 50%, magenta 50%

(f) Yellow 50%, cyan 50%

(g) Cyan 50%, magenta 50%

(h) Yellow 30%, magenta 40%, cyan 30%

(i) Yellow 30%, magenta 30%, cyan 30%

(j) Yellow 40%, magenta 40%, cyan 20%

(k) Yellow 10%, magenta 40%, cyan 50%

(l) Yellow 40%, magenta 10%, cyan 30%

(m) Yellow 10%, magenta 50%, cyan 40%

(n) Yellow 50%, magenta 20%, cyan 30%

(o) Yellow 30%, magenta 30%, cyan 30%, black 10%

For the glossiness, it was found that the 60° glossiness generally usedas an index of the specular gloss of printed matter did not agree withthe apparent glossy texture, even when it showed a high value. As aresult of extensive studies, it became clear that the glossiness at aview angle of 20° agreed with the apparent glossy texture. In theinvention, therefore, the glossiness at a view angle of 20° was taken asan index value.

Further, intermediate density is frequently used in an image such as anatural drawing or a portrait, so that the evaluation at intermediatedensity was employed in view of the effect in an actual printed image.

A: The average of respective patches was 50≦G₂₀.

B: The average of respective patches was 35≦G₂₀<50

C: The average of respective patches was G₂₀<35

(Test 4) Gloss Balance:

An ink cartridge for an ink jet printer, PX-V600 (manufactured by SeikoEpson Corporation) was filled with each ink, and allowed to stand atroom temperature for 6 months so that vibration is not given theretowith an ink feed opening directed downward. Then, each of the ink setsof Examples and Comparative Examples was mounted on the ink jet printer,PX-V600 (manufactured by Seiko Epson Corporation) according to an inkcartridge exchange method thereof, taking care not to give largevibration to the cartridge.

Using the patch pattern containing (a) to (j) used in test 3, printingwas repeated on EPSON Photo Paper <Gloss> (trade name: manufactured bySeiko Epson Corporation) at a resolution of 720 dpi until the ink in theink cartridge was consumed (ink end).

The 20° glossiness (G₂₀) of each patch of the resulting recorded matterwas measured by the use of a glossmeter, GM-268 (manufactured by KonicaMinolta Sensing, Inc.), and changes in glossiness from the start ofprinting to the ink end was examined for each patch. The changes in G₂₀(ΔG₂₀) were judged on the basis of the following criteria:

A: The maximum gloss difference from the start of printing to the inkend was ΔG₂₀<5.

B: The maximum gloss difference from the start of printing to the inkend was 5≦ΔG₂₀<10.

C: The maximum gloss difference from the start of printing to the inkend was 10≦ΔG₂₀.

(Test 5) Color Balance:

For the recorded matter obtained in test 4, the hue (L*, a*, b*) fromthe start of printing to the ink end was examined for each patch by theuse of a Gretag densitometer (manufactured by Gretag Macbeth), and fromthe results of the color difference (ΔE*) from just after the start ofprinting, judgment was made on the basis of the following criteria:

A: The maximum color difference from the start of printing to the inkend was ΔE*<3.

B: The maximum color difference from the start of printing to the inkend was 3≦ΔE*<6.

C: The maximum color difference from the start of printing to the inkend was 6≦ΔE*.

(Test 6) Storage Stability:

Fifty grams of each ink was put into an aluminum pack, and allowed tostand under the environment of 70° C. for 1 week. For whether foreignmatter (sediment) was generated or not after standing, further forchanges in physical properties (viscosity, surface tension, pH andparticle size) when no foreign matter was generated, judgment was madeon the basis of the following criteria:

A: No foreign matter was generated, and no physical properties werechanged.

B: No foreign matter was generated, but the physical properties weresomewhat changed.

C: Foreign matter was generated, or the physical properties weresignificantly changed.

(Test 7) Ejection Stability:

In printing in test 4, it was evaluated whether dot missing and flightdeflection of each ink occurred or not, and how many cycles of printercleaning which was conducted as a returning operation to normal printingwere required when they occurred. Judgment was made on the basis of thefollowing criteria:

A: No occurrence or one cycle of cleaning

B: Two to four cycles of cleaning

C: Five or more cycles of cleaning

The results of evaluations of the above test 3 to 7 are summarized inTable 4.

TABLE 4 Ink Set 1 2 3 4 5 6 7 8 Test 3 B A A B B A C C Test 4 B A A A AB B C Test 5 B A A A A C C C Test 6 A A A A A A A C Test 7 A A A A A A AC

As is apparent from Table 4, according to the ink set of the invention,there can be obtained images having excellent glossy texture to variousrecording media, particularly to glossy recording media, anddeterioration of image quality (deterioration of glossiness or colorbalance) caused by sedimentation of pigment particles in the pigment inkis small.

Further, the ink set of the invention is excellent in storage stability,and good in ejection stability even in a state in which pigmentparticles in the ink have been sedimented, leading to high reliability.Accordingly, the ink set of the invention is an ink set suitable for usein the ink jet recording system. Furthermore, deterioration of imagequality caused by sedimentation of pigment particles in the pigment inkis small even when no special device is made in an ink cartridge and thelike, so that an ink set having high flexibility is obtained.

The invention is not limited to the above-mentioned embodiments, and isalso applicable to applications, such as inks for writing things andinks for offset printing.

While the present invention has been described in detail and withreference to specific embodiments thereof, it will be apparent to oneskilled in the art that various changes and modifications can be madetherein without departing from the spirit and scope thereof.

This application is based on Japanese Patent Application No. 2005-094841filed Mar. 29, 2005, the contents thereof being herein incorporated byreference.

1. An ink set comprising at least the following pigment inks for an ink jet printer and different in color: a yellow ink, a magenta ink and a cyan ink, wherein the pigment inks each have a sedimentation rate, as indicated by equation (I), of 15% or less, and differences in the sedimentation rate between the pigment inks different in color are 4% or less: Sedimentation rate(%)=(S ₀ −S ₁)/S ₀×100  (I) wherein S₀ represents an “initial ink concentration”, which is an absorbance at a wavelength of λmax (or at 500 nm when λmax is not detected) at the time when UV spectral characteristics are measured for a 1000-fold diluted solution of the ink; and S₁ represents an “ink concentration after sedimentation”, which is an absorbance at a wavelength of λmax (or at 500 nm when λmax is not detected) at the time when UV spectral characteristics are measured for a 1000-fold diluted solution of a supernatant obtained by centrifuging the ink at 16500 G at 1.60×10⁷ g·sec, and wherein the ink set provides, when used for printing at a resolution of 1440 dpi, at the ink ratios (a) to (j) shown below, and at 70% duty for (a) to (g) and at an intermediate density of 50% duty for (h) to (j), print images of patch patterns having an average 20° glossiness (G₂₀) of all the patches of 35 or greater (a) Yellow 100% (b) Magenta 100% (c) Cyan 100% (d) Black 100% (e) Yellow 50%, magenta 50% (f) Yellow 50%, cyan 50% (g) Cyan 50%, magenta 50% (h) Yellow 30%, magenta 40%, cyan 30% (i) Yellow 30%, magenta 30%, cyan 30% (j) Yellow 40%, magenta 40%, cyan 20%
 2. An ink set comprising at least the following pigment inks for an ink jet printer and different in color: a yellow ink, a magenta ink and a cyan ink, wherein the pigment inks each have a sedimentation rate, as indicated by equation (I), of 15% or less, and differences in the sedimentation rate between the pigment inks different in color are 4% or less: Sedimentation rate(%)=(S ₀ −S ₁)/S ₀×100  (I) wherein S₀ represents an “initial ink concentration”, which is an absorbance at a wavelength of λ-max (or at 500 nm when λ-max is not detected) at the time when UV spectral characteristics are measured for a 1000-fold diluted solution of the ink; and S₁ represents an “ink concentration after sedimentation”, which is an absorbance at a wavelength of λ-max (or at 500 nm when λ-max is not detected) at the time when UV spectral characteristics are measured for a 1000-fold diluted solution of a supernatant obtained by centrifuging the ink at 16500 G at 1.60×10⁷ g·sec, and wherein the ink set provides, when used for printing at a resolution of 720 dpi, at the ink ratios (a) to (j) shown below, and at 70 duty for (a) to (g) and at an intermediate density of 50% duty for (h) to (j), print images of patch patterns having a maximum difference in average 20° glossiness (G₂₀) of all the patches from the start of printing to the ink end of smaller than
 10. 3. The ink set according to claim 2, wherein the print images show a color difference (ΔE*) from the start of printing to the ink end of less than
 6. 4. The ink set according to claim 1, wherein differences in average particle size between the pigment inks different in color are 30 nm or less.
 5. The ink set according to claim 1, wherein the pigment inks comprise pigment particles having an average particle size ranging from 50 to 150 nm.
 6. The ink set according to claim 1, wherein the yellow ink contains at least one pigment selected from the group consisting of C.I. Pigment Yellow 74, 109, 110, 128, 138, 147, 150, 155, 180 and 181, the magenta ink contains at least one pigment selected from the group consisting of C.I. Pigment Magenta 122, 202, 207, 209 and C.I. Pigment Violet 19, and the cyan ink contains at least one pigment selected from the group consisting of C. I. Pigment Blue 15, 15:1, 15:2, 15:3. 15:4 and
 16. 7. The ink set according to claim 2, wherein differences in average particle size between the pigment inks different in color are 30 nm or less.
 8. The ink set according to claim 2, wherein the pigment inks comprise pigment particles having an average particle size ranging from 50 to 150 nm.
 9. The ink set according to claim 2, wherein the yellow ink contains at least one pigment selected from the group consisting of C.I. Pigment Yellow 74, 109, 110, 128, 138, 147, 150, 155, 180 and 181, the magenta ink contains at least one pigment selected from the group consisting of C.I. Pigment Magenta 122, 202, 207, 209 and C.I. Pigment Violet 19, and the cyan ink contains at least one pigment selected from the group consisting of C. I. Pigment Blue 15, 15:1, 15:2, 15:3. 15:4 and
 16. 